"We gave beta-carotene supplements to people based on animal and some [preliminary] human studies suggesting they reduce lung cancer," he said.

"When we did the study in humans, smokers given beta-carotene got more lung cancer," he said.

Dr. Shields was referring to pivotal studies, conducted in the 1990s, of the effects of beta-carotene supplements on some 29,000 male smokers in Finland. They found that, compared with patients taking a placebo, those who consumed a daily beta-carotene supplement had a 18% higher rate of lung cancer.

Dr. Hardman's work, he said, "is a nice study that calls for more research. There needs to be a lot more understood" about how walnuts might prevent breast tumors.

"It might be worth it for epidemiologists to look at people," he added.

The value -- or risk -- of various foods is a common theme at this meeting. In other research:

Scientists at the University of Minnesota School of Public Health said eating well done and very well done meat may be associated with an increased risk of pancreatic cancer. A preference for very well done steak was associated with a 95% increase in the risk of the disease, although the hazard ratio just missed statistical significance.

For people with non-Hodgkin's lymphoma, a habit of drinking wine in the years before diagnosis may reduce the risk of death and relapse, according to researchers at Yale School of Public Health. The scientists said the finding may be controversial and needs to be replicated, but wine drinkers had 76% five-year survival compared with 68% of non-wine drinkers, while disease-free survival was 70% and 65% respectively.

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